Many industrial processes require the flow rates of fluids to be controlled, or different fluids to be mixed to achieve a desired output parameter. For example, many industrial applications require a flow of water at a particular temperature, and this is often achieved by mixing a flow of hot water with a flow of cold water to achieve a flow at the desired temperature. These processes can typically operate throughout the working day, and in some cases would operate around the clock.
Control valves that are used to control flow rates or fluid mixing in these industrial processes are often required to operate continuously, frequently adjusting the position of valve components to ensure that the desired output parameter is maintained.
Most valves are manufactured to have very close tolerances between the sealing surfaces of the valve components to minimize leakage. Such close tolerances can produce appreciable friction between the sealing surfaces of the valve components, and the friction causes drag when the valve components move relative to one another.
The combination of close tolerances, the associated friction and continuous operation with frequent valve position changes can produce unacceptable levels of wear in the valve components and in the actuators and any gearboxes or mechanisms used to move the valve components.
The drag caused by friction between the valve components can mean that the actuators of the control valves need to be relatively powerful, especially in cases where it is important to control the fluid flow or mixing ratios to a close tolerance and where rapid valve adjustments are required. Similarly gearboxes or other mechanisms used to move the valve components need to be robust to ensure reliability.
Frictional drag can also slow down the response time of a control valve. And similarly the friction and/or “stiction” can reduce the accuracy in positioning the valve's moving components.
Water mixers for hand-basins, showers or baths can also experience these problems.
These factors mean that control valves are often relatively expensive, and they can require maintenance or overhauls at frequencies that are too often. Also, the control valves can have response times that are too slow, and their accuracy as flow controllers or mixing valves can be poor.